1. Field of the Invention
The invention concerns a motion sensor for measuring a patient's activity level of the type having a fluid-tight, biocompatible housing, a number of electrodes coupled to the housing and the housing containing a fluid. Furthermore, the invention relates to an implantable cardiac pacemaker incorporating such a motion sensor.
2. Description of the Prior Art
Pacemakers are used to pace the beating frequency of the heart. They have been of greatest importance for helping individuals suffering from various heart diseases and/or failures to live rather normal lives. Pacemakers function by generating electrical pulses, which stimulate the heart. In order to pace the heart in a correct way, the specific pulse pattern of the individual heart must be known. It is advantageous to detect external parameters, such as the motion of the patient, in order for the pacemaker to stimulate the heart as correctly as possible.
Normally, the motion of the patient is measured by some kind of motion sensor, which is connected to the pacemaker. U.S. Pat. No. 5,755,741 discloses an implantable sensor providing an indication of movement and orientation of a patient. The sensor may be connected to a pacemaker. The sensor has a cylindrical enclosure having a central electrode within a cavity of the enclosure, and one or more peripheral electrodes within the cavity. An electrolytic fluid is positioned in the enclosure so that movement of the sensor results in variations of the amount of the fluid between the central electrode and one or more of the peripheral electrodes. An alternating current applied to the electrodes will produce an output voltage signal, which varies in accordance with the movements of the fluid (and the sensor).
U.S. Pat. No. 5,833,713 discloses an accelerometer-based, multi-axis physical activity sensor for use with a rate-responsive pacemaker. A piezoelectric polymer film is adhered to the surface of an electrically conductive substrate on the sensor. In response to bodily accelerations the piezoelectric film produces an output signal.
U.S. Pat. No. 5,233,984 discloses a multi-axis sensor, for example connected to a pacemaker, for measuring a patient's activity level. The sensor has a hermetically sealed, fluid-tight, biocompatible housing with a number of electrodes coupled to the sides of the housing, and a central electrode positioned within the housing. An electrically conductive electrolyte fills about half of the housing, allowing voltage changes, due to motion of the sensor, between the central electrode and the other electrodes to be monitored.
Also, U.S. Pat. No. 4,869,251 discloses a pacemaker having a sensor for detecting inertial and/or rotational movements of a patient. The sensor has a hollow member, with at least one freely moveable member therein generating a mechanical vibration upon movement of the patient. A transducer generates an electric signal corresponding to the mechanical vibrations. It is stated that the hollow member may be filled with a fluid and/or a number of particles producing pressure to the walls of the hollow member. It is also stated that the sensitivity of the sensor may be different in different directions. For example, the moveable element may be a magnetic dipole, and the transducer may be one or more coils arranged around the hollow member, whereby a current is generated in the coils when the moveable element changes position in the interior of the hollow member. Depending on configuration, however, the mobility of the movable element may degrade due to mechanical wear.
Such known motion sensors generally rely on moving mechanical components, and therefore they may be difficult to miniaturize further. This is a problem, because the size of the motion sensor (and pacemakers) advantageously is as small as possible, in order to disturb the biological system into which they are implanted as little as possible. Furthermore, as mentioned above, mechanically based sensors may suffer from mechanical wear, which can lead to their failure.